1. Field of the Invention
This invention relates to a position detection apparatus, used in e.g. machine tools or industrial machines, for detecting the position which has been varied due to linear movements or rotational movements. This application claims priority of Japanese Patent Application No. 2002-190773, filed on Jun. 28, 2002, the entirety of which is incorporated by reference herein.
2. Description of Related Art
Among the position detection apparatus, used in e.g. machine tools or industrial machines, for detecting changes in the positions caused by e.g. linear movements, there are those disclosed in Japanese Laying-Open Patent Publications Nos.S50-99564, S63-177019 and H1-152314.
Such a position detection apparatus 120, shown in FIG. 12, includes one absolute track, referred to below as an ABS track 121, and a detection unit for the ABS track, referred to below as an ABS detection unit 124, having n heads for the absolute track, referred to below as ABS heads 123-1, 123-2, . . . , 123-n. It should be noted that the ABS heads 123-1, 123-2, . . . , 123-n, if collectively referred to, are termed ABS heads 123-1 to 123-n. 
The ABS track 121 is a pattern of micro-sized areas represented by [0] and by [1], arrayed in accordance with the degree-n cyclic code. The micro-sized areas represented by [0] and by [1] differ from each other as to physical properties. For example, if the position detection apparatus 120 detects the position of the ABS detection unit 124 by exploiting the magnetism, micro-sized areas represented by [1] are magnetized, while those represented by [1] are not magnetized.
The ABS detection unit 124 is movable is movable along the longitudinal direction of the ABS track 121, indicated by arrow Y in the drawing. The ABS heads 123-1 to 123-n are provided facing the ABS track 121. Moreover, the ABS heads 123-1 to 123-n are provided at an interval xcex from one another, where xcex is the pitch of the micro-sized areas.
The ABS head 123-1 includes magnetizable devices 123-1a and 123-1b. The magnetizable devices may, for example, be MR devices. The ABS head 123-1 also includes a wiring and is in operation by the current flowing therein. Meanwhile, the ABS heads 123-2 to 123-n are of the same structure as the ABS head 123-1.
In the above-described position detection apparatus 120, in which the ABS track 121 is of a pattern having an array conforming to the degree n cyclic code, the n-bit codes, detected by the ABS detection unit 124, referred to below as ABS values, assume different values, as the position of the ABS detection unit 124 relative to the ABS track 121 is changed, despite the fact that the ABS track provided is one track. Thus, the position detection apparatus 120 is able to detect the position of the ABS detection unit 124 relative to the ABS track 121, based on the ABS value as detected by the ABS detection unit 124.
With the position detection apparatus 120, in which the ABS track provided is one track, it is possible to reduce the size along the width-wise direction of the ABS track 121, indicated by arrow Z in the drawing. That is, the position detection apparatus 120 can be reduced in size.
In the above-described position detection apparatus 120, there is raised a demand for higher definition and higher resolution in order to measure position changes of the ABS detection unit 124 relative to the ABS track 121 more precisely. Among the methods for further improving the definition and the resolution, there is a method for decreasing the pitch of the micro-sized areas. When the pitch of the micro-sized areas is reduced, it becomes necessary to diminish the gap between the ABS heads 123-1 to 123-n. 
However, when the gap between the ABS heads 123-1 to 123-n is diminished, the ABS heads 123-1 to 123-n need to be arranged congested. When the ABS heads 123-1 to 123-n are arranged congested, the state of FIG. 13 results for n=4, as an example, as a result of which the magnetizable devices 123-1a to 123-4a and 123-1b to 123-4b, provided to the ABS heads 123-1 to 123-4, are congested.
The ABS heads 123-1 to 123-n are in operation by the current flowing therein. For example, the respective ABS heads 123-1 to 123-n are determined to be facing the micro-sized areas represented by [1] or the micro-sized areas represented by [0] by measuring the resistance values thereof. That is, the current flows in the respective ABS heads 123-1 to 123-n. 
Thus, when the ABS heads 123-1 to 123-n are congested, the intensity of the current flowing per unit area is increased in an area of high degree of congestion of the magnetizable devices provided to the ABS heads 123-1 to 123-n, with the result that the heat evolution in the area is increased. If, in the area of high degree of congestion of the magnetizable devices provided to the ABS heads 123-1 to 123-n, the amount of heat evolution per unit area is increased, the ABS heads 123-1 to 123-n are deteriorated in characteristics, or are destroyed.
Moreover, if the ABS heads 123-1 to 123-n are congested, the wiring provided to these ABS heads 123-1 to 123-n is concentrated, with the result that the wiring needs to be reduced in diameter, thus increasing the possibility of line breakage or the current leaking to the neighboring ABS heads.
If the ABS heads 123-1 to 123-n are deteriorated in characteristics or destroyed, or if line breakage of the wiring or the current leakage to the neighboring ABS heads occurs, the possibility of malfunctioning of the position detection apparatus 120 increases, and hence the position detection apparatus 120 is deteriorated in operational reliability.
In view of the above depicted status of the art, it is an object of the present invention to provide a position detection apparatus in which, when the pitch of the micro-sized areas is diminished to improve the definition and resolution, it is possible to realize high operational reliability.
The present invention provides a position detection apparatus comprising an absolute track composed of a string of a number of micro-sized areas each indicated by a code of [0] and a number of micro-sized areas each indicated by a code of [1] in accordance with a maximum length sequence generated by a degree-n primitive polynominal, n being a natural number not less than 2, and a detection unit for the absolute track movable relative to the absolute track, the detection unit including n heads for the absolute track, arrayed facing the absolute track, for detecting the physical properties of the absolute track. The micro-sized areas each indicated by a code of [1] differ in physical properties from the micro-sized areas each indicated by a code of [0]. The n heads for the absolute track are arrayed under a condition satisfying the equations 1 and 2 if n is an even number and under a condition satisfying the equations 1 and 3 if n is an odd number:
xcex1=mxcex, m being an integer not less than 2xe2x80x83xe2x80x83(1) 
xcex1xe2x89xa0k(2n/2+1)xcex, k being a natural numberxe2x80x83xe2x80x83(2) 
xcex1xe2x89xa0k(2n+1)xcexxe2x80x83xe2x80x83(3) 
where xcex is a pitch of the micro-sized areas and xcex1 is the separation between neighboring ones of the n heads for the absolute track.
With the position detection apparatus, the n ABS heads are arranged under the conditions satisfying the equations (1) and (2) or under the conditions satisfying the equations (1) and (3) when n is even or odd, respectively. By arraying the n ABS heads under these conditions, the separation between neighboring ones of the ABS heads in the position detection apparatus of the present invention can be 2xcex or larger.
With the position detection apparatus, in which the separation xcex1 of the heads for the absolute track (ABS heads) may be set to 2xcex or larger, it becomes unnecessary to reduce xcex1 even when the pitch of the micro-sized areas is reduced with a view to achieving high definition and high resolution.
Thus, with the position detection apparatus, in which pitch of the micro-sized areas is reduced, there is no risk of congestion of the n heads for the absolute track.
Since the n heads for the absolute track are not congested, it is possible with the position detection apparatus to reduce heat evolution from the area of congestion of the n heads for the absolute track.
Moreover, since the n heads for the absolute track are not congested, there is no risk of the wiring provided to the respective heads for the absolute track becoming congested in a preset area.
Since the wiring provided to the respective heads for the absolute track are not congested in a preset area, it is possible with the position detection apparatus to prevent the wiring provided to the respective heads for the absolute track from becoming complex, thus assuring facilitated designing of the detection unit for the absolute track.
Additionally, since the wiring provided to the respective heads for the absolute track are not congested in a preset area, it is possible with the position detection apparatus to use thick wiring. Since the thick wiring may be used for the heads for the absolute track, it is possible with the position detection apparatus to prevent the rupture of the wiring breakage or current leakage to the neighboring heads for the absolute track.
Thus, the present position detection apparatus is improved in operational reliability.